Solve the 3-variable system by substitution

Question

{x-y-3z=1, x-z=-4, 3x=-15}

Answer 1

x=-5, y=-3, z = -1

3x=-15, x = -15/3 = -5

x-z=-4, z = -5 +4 = -1

x-y-3z = 1

-5 - y +3 = 1

y = -2-1 = -3